This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
Co-pol co-polarization, co-polarized
CDF cumulative distribution function
CQI channel quality indicator
DL downlink
ECGI E-UTRAN cell global identifier
eNB evolved Node B/base station in an E-UTRAN system
E-UTRAN evolved UTRAN (LTE)
FDD frequency division duplex
ID identification
InH indoor hotspot
IMSI international mobile subscriber identity
ITU International Telecommunications Union
LA location area
LOS loss of signal
LTE long term evolution
LTE-A long term evolution advanced
MCC mobile country code
MME mobility management entity
Nrx Number of Receive Antennas
PLMN public land mobile network
PMI precoder matrix index
RA routing area
RAN radio access network
RMa rural macro
RNC radio network controller
RRC radio resource control
SNR signal-to-noise ratio
SRS sounding reference signal
TDD time division duplex
TS technical standard
UE user equipment
UL uplink
UMa urban macro
UMi urban micro
UTRAN universal terrestrial radio access network
XPOL cross polarization, cross-polarized
Antenna deployments for wireless networks may consist of antenna arrays containing a plurality of co-polarized (co-pol) elements, such as a plurality of elements with vertical-only polarization or a plurality of elements with horizontal-only polarization. In wireless networks employing beamforming, these co-pol elements are phased to form a single logical antenna having a radiation pattern that can be controlled in the azimuthal dimension. A similar concept is often employed inside antenna panel arrays containing a plurality of cross-polarized (XPOL) elements for the purpose of combining several antenna elements into one logical antenna port. For example, FIG. 1 illustrates a physical XPOL antenna panel 100 comprised of multiple elements α1, α2 . . . αQ arranged at +45° polarization with reference to the horizontal plane, as well as multiple elements αQ+1, αQ+2 . . . α2Q arranged at −45° polarization with reference to the horizontal plane. The +45° elements α1, α2 . . . αQ are phased to form a single logical +45° antenna P1. Likewise, the −45° elements αQ+1, αQ+2 . . . α2Q are phased to form a single logical −45° antenna P2. The XPOL antenna panel 100 of FIG. 1 forms a single logical antenna port for each polarization, such that the logical antenna P1 forms a single first logical antenna port for the +45° polarization, and the logical antenna P2 forms a single second logical antenna port for the −45° polarization.
The XPOL antenna panel 100 therefore forms a logical two-port XPOL array. The logical +45° antenna P1 is implemented using a first input port 102, and the logical −45° antenna P2 is implemented using a second input port 104. The first input port 102 is connected to a first input of a first multiplier 106, a first input of a second multiplier 108, and a first input of a third multiplier 110. A first transmit (Tx) weight f1 is applied to a second input of the first multiplier 106. A second Tx weight f2 is applied to a second input of the second multiplier 108. Likewise, a third Tx weight fQ is applied to a second input of the third multiplier 110. An output of the first multiplier 106 is connected to the element α1, an output of the second multiplier 108 is connected to the element α2, and an output of the third multiplier 110 is connected to the element αQ.
The second input port 104 of the XPOL antenna panel 100 is connected to a first input of a fourth multiplier 112, a first input of a fifth multiplier 114, and a first input of a sixth multiplier 116. A fourth Tx weight fQ+1 is applied to a second input of the fourth multiplier 112. A fifth Tx weight fQ+2 is applied to a second input of the fifth multiplier 114. Likewise, a sixth Tx weight f2Q is applied to a second input of the sixth multiplier 116. An output of the fourth mixer 112 is connected to the element αQ+1, an output of the fifth multiplier 114 is connected to the element αQ+2, and an output of the sixth multiplier 116 is connected to the element α2Q.
In the case of XPOL as well as co-pol antenna panels, electrical phasing (through the proper selection of the Tx weights f1 . . . f2Q) may be used to control the vertical plane radiation patterns of the logical antennas in the antenna panel so as to create a specific pattern in the elevation dimension. This technique is called electrical beam tilt. In some situations, mechanical beam tilt may be employed in addition to, or in lieu of, electrical phasing to optimize cell coverage. Beam tilt is often used to increase the overall gain of antenna panels and also to improve coverage of macrocells when antenna panels are mounted on structures situated at a relatively high elevation above ground, such as atop a high tower or tall building. However, mechanical beam tilt is slow to adapt to changing conditions and both mechanical and electric phasing mentioned above apply the same downtilt to all users being transmitted to at a given time. But regardless of whether mechanical beam tilt, electrical beam tilt, or some combination thereof is implemented, the resulting elevation pattern may not be optimum for every user in a given cell. Hence there is a need for a method and apparatus to perform per-user beamforming in both the azimuth and elevation dimensions